1. Field of the Invention
The present invention relates to an image display apparatus for displaying data in computers, facsimiles and the like, and more particularly to an apparatus for forming and displaying an image on a repetitively usable image bearing web in the form of a belt.
2. Description of the Prior Art
In the prior art, a cathode ray tube (CRT) is currently used as an image display apparatus. Where a CRT is utilized to display a still image such as a letter, a figure or the like, a random access memory (RAM) as an IC memory is generally used together with the CRT in connection with picture elements of a picture to store the information of image which will be read out for display.
In general, it may often be required that the image display apparatus displays the previously displayed image repetitively. In order to fulfill such a requirement, the image display apparatus including the CRT is provided with a RAM capable of storing data for a plurality of pictures to allow a new picture to be displayed without the necessity of clearing the RAM of the data for the picture currently on display. Thus, the respective pictures can be re-displayed at any time by reading out the information which has been stored in the RAM. The RAM required to store the information of all the pictures must have an extremely great capacity and is then substantially expensive. Also, the image display apparatus using the CRT has another problem of unsatisfactory resolving power.
An image display apparatus of thermal recording type is known as having a great capacity with respect to information of pictures. This apparatus comprises a reversible heat-sensitive recording web, for example, of Ag.sub.2 HgI.sub.4 which is a compound of silver, mercury and iodine, and a thermal head for recording an image on the heat-sensitive recording web. Such a compound, Ag.sub.2 HgI.sub.4, changes in color in accordance with the change of temperature and has a hysteresis with respect to temperature. Therefore, if an image is recorded on a belt-shaped film containing Ag.sub.2 HgI.sub.4 by the use of the thermal head, the image can be displayed by maintaining the temperature of the film by the use of a planar heater and erased by cooling the film at room temperature or by the use of a cooler. Thus, the film can repetitively be utilized.
There is also known an image display apparatus of electrophotographic type in which an electrophotographic type photosensitive member is scanned by a light beam which has been modulated in accordance with image signals, to form a toner image on the photosensitive member. This toner image will be displayed. Thereafter, the toner image can be removed by the use of subsequent light beam application so that the photosensitive member can repetitively be used.
The image display apparatus of either heat-sensitive recording type or electrophotographic type has an advantage in that it has an extremely great capacity of display and a good resolving power. For example, if there are eight dots per one millimeter and when there is displayed an image having dimensions of 200 mm.times.300 mm, picture elements as many as three million, eighty hundred and fourty thousand can be displayed. If there are 16 dots per one millimeter, the number of picture elements reaches fifteen million, three hundred and sixty thousand. In order to redisplay the previously displayed image in such an image display apparatus, a random access memory (RAM) is similarly used to store all the display information, that is, all picture elements of one entire picture entirely. Thus, the RAM must have a great capacity, resulting in an extremely expensive structure increased in size. For example, if LSI memories of 64 kilo-bit are used to store the information of fifteen million, three hundred and sixty thousand of picture elements, it is required that the number of LSI memories is 235. Considering memory drive circuits and associated power supply and others, the image display apparatus will be considerably bulky and costly.
The prior art image display apparatus of thermal recording type or the like involves another problem that a speed for which an image is being displayed is not very high. This is also one of various causes of disadvantages produced when the aforementioned memories are used. For example, the thermal recording type image display apparatus is adapted to write the respective picture elements on the image bearing web successively and exactly while the latter is mechanically moving. Accordingly, the complete display of a picture requires a period of time in the range of a few seconds to several tens of seconds. Of course, this period cannot be reduced even when the previously displayed image is re-displayed after it has been once stored in the memory.